In wireless communication systems, transmission techniques involving multiple antennas are often categorized as open-loop or closed-loop, depending on the level or degree of channel response information used by the transmission algorithm. Open-loop techniques do not rely on the information of the spatial channel response between the transmitting device and the receiving device. They typically involve either no feedback or the feedback of the long term statistical information that a base unit may use to choose between different open loop techniques. Open-loop techniques include transmit diversity, delay diversity, and space-time coding techniques such as the Alamouti space-time block code.
Closed-loop transmission techniques utilize knowledge of the channel response to weigh the information transmitted from multiple antennas. To enable a closed-loop transmit array to operate adaptively, the array must apply transmit weights derived from channel state information (CSI) between each of the transmitter's antennas and each of the receiver's antennas which may include the channel response, its statistics or characteristics, or a combination thereof. One method to obtain the CSI is through a feedback channel between the receiver and the transmitter. This CSI feedback channel may consist of any technique known in the art such as analog feedback of the channels, analog feedback of the statistics (e.g., the covariance matrix or the eigenvector/eigenvectors), quantized feedback of the statistics, quantized feedback of the channel, or codebook feedback.
In order to calculate any of the CSI feedback needed for closed-loop operation, the transmitter must have a mechanism that enables the receiver to estimate the channel between the transmitter's antennas and the receiver's antennas. The channel estimation between the transmit and the receive antennas is also needed for the calculation of non-spatial feedback information including modulation and coding rate (MCS), sub-band selection that are applicable for both open-loop and closed-loop transmissions. The usual mechanism to enable the channel estimation by the receiver is by the transmitter sending pilot signals (also known as reference symbols) from each of the transmit antennas which essentially sound the channel. A pilot signal (also known as reference symbols or RSs) is a set of symbols known by both the transmitter and receiver. The mobile would then use the pilot signals to compute channel estimates which can then be used to determine the CSI feedback. Typical methods for pilot transmission use a frequency-domain pilot sequence and possibly some spreading of the pilot signal with repetition or a Walsh code. The frequency-domain pilot sequence would be different for each unique transmitter and the sequences are typically designed to have a low correlation between transmitters to keep interference at a low level. The frequency-domain sequence can be made to be orthogonal between a limited set of base stations, but to do so requires a substantial increase in the pilot density in frequency. The Walsh codes if properly used can provide some orthogonality to transmitters, but are limited to a few orthogonal codes which are insufficient to keep interference at a minimum when there are many interferers. Also the Walsh codes are limited to being only orthogonal between the small set of transmitters and cannot be quasi orthogonal to a much larger set of transmitters (where quasi-orthogonality means a guaranteed level of interference suppression such as 6.0 dB).
While the above-techniques for pilot signal transmission may provide a mechanism for pilot signal transmission for use in CSI determination, the methods are not optimized for multi-transmitter operation which needs both orthogonal and quasi-orthogonal pilot signals. Thus there is a need for an improved pilot signal design without the need of increasing the pilot density in frequency for enabling optimal CSI determination at a receiver.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.